The TV was �30, which the people originally paid.
Having realised the TV should have been �25, the assistant refunds the people �3, which means the people have paid �27 for a �25 TV.
The riddle is deliberately set out to 'trick' your brain into trying to make a mathematical equation that does not add up.
�30 (original price) minus �5 (discounted amount) equals �25 plus �3 (refunded amount) equals �28 plus �2 (the stolen amount the people think have gone toward the price of the TV) equals �30.